1. Field of the Invention
The invention relates generally to antennas and more particularly to a dual frequency, vertical antenna.
2. Description of the Prior Art
Vertical antennas have been used for many years to radiate a radio frequency signals. These antennas commonly radiate (and receive) the signal from a dipole having a horizontal ground plane and a vertical mast extending upward from the ground plane. The signal is vertically polarized and radiate in a direction approximately perpendicular to the mast, decreasing to a null in the direction that the mast extends. The ground plane is typically a horizontal surface area having another function as a wetland, an equipment enclosure, or a vehicle body. Because half of the dipole structure is in the ground plane, the vertical antenna has an advantage of being half the size of other antenna types. A further advantage is that the structure of a vertical antenna can be simple and inexpensive to construct.
Commercial Global Positioning System (GPS) receivers are now used in many navigation, tracking, and timing applications to receive a GPS signal at approximately 1.575 GHz from one or more GPS satellites and to provide a GPS based location. The system, currently including a constellation of 21 to 24 GPS satellites, is controlled and maintained by the United States Government. A GPS antenna receives the GPS satellite signals and provides an electronic GPS signal for the GPS receiver. The GPS receiver measures ranges to four GPS satellites simultaneously where each satellite has a line of sight to the GPS antenna and determines the GPS location. The inherent GPS location accuracy is approximately 20 meters. However, a selective availability (SA) is currently in place that degrades the actual accuracy to the GPS location to the range of 50 meters to 300 meters.
Differential GPS receivers, termed "DGPS" receivers, use differential corrections to improve the accuracy of the GPS based location. These differential corrections are determined by comparing the GPS based location determined by a GPS receiver with a surveyed location. Certain FM stations broadcast these differential corrections in a subcarrier of the FM broadcast signal. The DGPS receiver receives the FM signal and uses the corrections to enhance the location accuracy to a range between 10 meters and a few centimeters.
GPS receivers are used in tracking systems to provide the location of a mobile platform. The platform may be a car, truck, or bus on land, a ship or boat on water, or an airplane or spacecraft above the Earth's surface. A radio on the mobile platform transmits the GPS-based location of the platform to a base station in a radio signal.
A dual frequency antenna has a advantage of using less space and costing less than two separate antennas. Further, a vertical antenna typically uses less space and is inherently simpler and lower cost than other types of antennas. Unfortunately, little work has been done on vertical GPS antennas because of well-known problems that the orbits of the GPS satellites will sometimes place the satellites in the null direction of the antenna and that the vertical polarization of the antenna reduces the received GPS signal strength to approximately one-half the signal strength that is available from a circularly polarized antenna.
Another problem in a design for a dual frequency, vertical antenna is that the extent and structure of the ground plane may change the tuning of the antenna at the higher of the two frequencies radiated by the antenna. In order to minimize the effect of the ground plane it is desirable to radiate the higher of the two frequencies from the upper portion of the mast.
Several patents disclose dual frequency, vertical antennas. Unfortunately, such the antennas that have been disclosed have sacrificed the inherent simplicity and low cost of the vertical antenna.
There is a need for a simple dual frequency, vertical antenna to radiate a higher signal frequency, such as a GPS signal frequency, from an upper portion of a mast and simultaneously to radiate a lower signal frequency.